BMR question...

So I have been reading all of this info on the BMR and how many things affect it.  Also, how by exercising and having more lean muscle with up your metabolism just like no exercise will lower it.  Also that when you have more muscle you are burning calories quicker....that being said how can we...I really be sure of how many calories to eat if I can't really be sure of how many calories I burn.  I mean...what if I do have a fast metabolim and don't know it and I'm starving myself or what if I have a slow one and I'm overeating...can't both make you gain weight, ie starvation mode.

So confused!!

Would purchasing a HRM be good for this?  That way I can really see what I am burning. 

The differences in the amount of calories with an extremely accurate reading and an estimate reading (like the tools on cc) are really not that much different.  This bothered me when I first started too, but once I realized that nothing has to be exact it didn't upset me as much.  You will also never have a precise number of calories you eat unless you weigh everything and don't do anything with recipes also.  Sometimes you will have to estimate and thats ok!  Good luck to you.